In a variety of technical fields it frequently occurs that heat is generated at a certain point and has to be transferred to another point. This is not only the case if heat is generated on purpose (for example for heating purposes), but also if heat is only generated as some kind of a by-product. In the latter case, one typically talks about the generation of waste heat.
A particular example of such a generation of waste heat occurs in the field of electronics and illumination systems (in particular in the case of power electronics and illuminating systems, using LEDs (light emitting diodes)). Here, it frequently occurs that a substantial amount of thermal energy is generated in a comparatively small volume. Furthermore, in particular in case of semiconductors and LEDs, the generated waste heat can only be removed through very small interfaces, for example on only one side of a component, since, for example, the light generated by a LED is supposed to be emitted into a dihedral angle that is as large as possible. In other words, the waste heat power density is quite high and the thermal fluxes that have to be managed can be substantial, as well.
To make things worse, in particular in the field of semiconductors and LEDs, the electronic components are very sensitive to overheating. As an example, if a power LED is operated at a temperature above 65° C. (approximately), its lifetime will substantially degrade. This is also known as the Arrhenius Exponential Law, suggesting that if the temperature is raised by 10° C., a given chemical reaction will proceed approximately twice as fast. Regarding electronic components, this will result in a rule of thumb saying that for every 10° C. increase in temperature, the risk of failure for the respective electronic component doubles.
Therefore, it is a standard move to provide systems that are generating waste heat with a cooling system that is intended to keep the temperature as low as possible with the underlying aim to prolong the lifetime of the electronic components and hence of the resulting device.
However, this necessitates a cooling system that is designed sufficiently large, keeping in mind that cooling systems normally become more effective if the tolerated temperature level of the heat generating device is higher since as a consequence the resulting temperature difference to the ambient temperature becomes larger. Therefore, to be able to lower the internal temperatures, one has to use a cooling system that has a disproportionately larger dimension and is hence more expensive.
The problem of dimensioning a cooling system is particularly dominant, if the amount of waste energy that is generated is not constant and shows only some comparatively large peaks within a base load of generated waste heat. To be able to maintain the temperature at a constant level in such a case, one has to design the cooling capacity of the cooling system according to the waste-heat peaks, which will result in a disproportionately dimensioned cooling system and hence in disproportionately high cost.
It has been already proposed in the state-of-the-art to allow some temperature fluctuations in such a case. Even if the lifetime of the electronic components is shortened by a certain amount due to the increased temperature level, the total cost over a certain time span can still be lower, since the cost for the cooling system can be disproportionately lower. In such a case, the cooling system runs at maximum load if a waste heat-peak occurs. Since the cooling system is “under-dimensioned” with respect to the waste heat generated during such a peak, the temperature of the electronic component will increase (slightly) and decrease once again after the waste heat-peak is over and hence the generated waste heat is lower than the maximum cooling rate of the arrangement again.
Unfortunately, this approach gives rise to yet another problem. In particular in power electronics, typically a variety of different parts with different thermal expansion coefficients are fixedly connected to each other (for example a flat pack chip or ball grid array chip that is soldered to a certain substrate). Therefore, temperature changes will result in a varying mechanical load. If such a thermal stress cycle is repeated very often, this will typically result in a mechanical damage or failure of the parts involved (for example solder points, the chip or the substrate). Of course, such a damage is highly undesired as well.
Studies have shown that by reducing the quantity and/or magnitude of the temperature swings experienced by the individual electronic components of an electronic assembly, it is possible to reduce the thermal stresses on the components and thus increase the lifetime of the system, even if this reduction of temperature swings is at the expense of a slightly increased average temperature.